Lib/whrandom.py

   1 """Wichman-Hill random number generator.
   2
   3 Wichmann, B. A. & Hill, I. D. (1982)
   4 Algorithm AS 183:
   5 An efficient and portable pseudo-random number generator
   6 Applied Statistics 31 (1982) 188-190
   7
   8 see also:
   9         Correction to Algorithm AS 183
  10         Applied Statistics 33 (1984) 123
  11
  12         McLeod, A. I. (1985)
  13         A remark on Algorithm AS 183
  14         Applied Statistics 34 (1985),198-200
  15
  16
  17 USE:
  18 whrandom.random()       yields double precision random numbers
  19                         uniformly distributed between 0 and 1.
  20
  21 whrandom.seed(x, y, z)  must be called before whrandom.random()
  22                         to seed the generator
  23
  24 There is also an interface to create multiple independent
  25 random generators, and to choose from other ranges.
  26
  27
  28 Translated by Guido van Rossum from C source provided by
  29 Adrian Baddeley.
  30
  31
  32
  33 Multi-threading note: the random number generator used here is not
  34 thread-safe; it is possible that nearly simultaneous calls in
  35 different theads return the same random value.  To avoid this, you
  36 have to use a lock around all calls.  (I didn't want to slow this
  37 down in the serial case by using a lock here.)
  38 """
  39
  40 class whrandom:
  41         def __init__(self, x = 0, y = 0, z = 0):
  42                 """Initialize an instance.
  43                 Without arguments, initialize from current time.
  44                 With arguments (x, y, z), initialize from them."""
  45                 self.seed(x, y, z)
  46
  47         def seed(self, x = 0, y = 0, z = 0):
  48                 """Set the seed from (x, y, z).
  49                 These must be integers in the range [0, 256)."""
  50                 if not type(x) == type(y) == type(z) == type(0):
  51                         raise TypeError, 'seeds must be integers'
  52                 if not (0 <= x < 256 and 0 <= y < 256 and 0 <= z < 256):
  53                         raise ValueError, 'seeds must be in range(0, 256)'
  54                 if 0 == x == y == z:
  55                         # Initialize from current time
  56                         import time
  57                         t = long(time.time() * 256)
  58                         t = int((t&0xffffff) ^ (t>>24))
  59                         t, x = divmod(t, 256)
  60                         t, y = divmod(t, 256)
  61                         t, z = divmod(t, 256)
  62                 # Zero is a poor seed, so substitute 1
  63                 self._seed = (x or 1, y or 1, z or 1)
  64
  65         def random(self):
  66                 """Get the next random number in the range [0.0, 1.0)."""
  67                 # This part is thread-unsafe:
  68                 # BEGIN CRITICAL SECTION
  69                 x, y, z = self._seed
  70                 #
  71                 x = (171 * x) % 30269
  72                 y = (172 * y) % 30307
  73                 z = (170 * z) % 30323
  74                 #
  75                 self._seed = x, y, z
  76                 # END CRITICAL SECTION
  77                 #
  78                 return (x/30269.0 + y/30307.0 + z/30323.0) % 1.0
  79
  80         def uniform(self, a, b):
  81                 """Get a random number in the range [a, b)."""
  82                 return a + (b-a) * self.random()
  83
  84         def randint(self, a, b):
  85                 """Get a random integer in the range [a, b] including both end points.
  86                 (Deprecated; use randrange below.)"""
  87                 return self.randrange(a, b+1)
  88
  89         def choice(self, seq):
  90                 """Choose a random element from a non-empty sequence."""
  91                 return seq[int(self.random() * len(seq))]
  92
  93         def randrange(self, start, stop=None, step=1, int=int, default=None):
  94                 """Choose a random item from range([start,] step[, stop]).
  95                 This fixes the problem with randint() which includes the
  96                 endpoint; in Python this is usually not what you want.
  97             Do not supply the 'int' and 'default' arguments."""
  98                 # This code is a bit messy to make it fast for the
  99                 # common case while still doing adequate error checking
 100                 istart = int(start)
 101                 if istart != start:
 102                         raise ValueError, "non-integer arg 1 for randrange()"
 103                 if stop is default:
 104                         if istart > 0:
 105                                 return int(self.random() * istart)
 106                         raise ValueError, "empty range for randrange()"
 107                 istop = int(stop)
 108                 if istop != stop:
 109                         raise ValueError, "non-integer stop for randrange()"
 110                 if step == 1:
 111                         if istart < istop:
 112                                 return istart + int(self.random() *
 113                                                    (istop - istart))
 114                         raise ValueError, "empty range for randrange()"
 115                 istep = int(step)
 116                 if istep != step:
 117                         raise ValueError, "non-integer step for randrange()"
 118                 if istep > 0:
 119                         n = (istop - istart + istep - 1) / istep
 120                 elif istep < 0:
 121                         n = (istop - istart + istep + 1) / istep
 122                 else:
 123                         raise ValueError, "zero step for randrange()"
 124
 125                 if n <= 0:
 126                         raise ValueError, "empty range for randrange()"
 127                 return istart + istep*int(self.random() * n)
 128
 129
 130 # Initialize from the current time
 131 _inst = whrandom()
 132 seed = _inst.seed
 133 random = _inst.random
 134 uniform = _inst.uniform
 135 randint = _inst.randint
 136 choice = _inst.choice
 137 randrange = _inst.randrange